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Antiphospholipid (aPL)/anti-␤ 2 glycoprotein I (anti-␤ 2 GPI) antibodies stimulates tissue factor (TF) expression within vasculature and in blood cells, thereby leading to increased thrombosis. Several cellular receptors have been proposed to mediate these effects, but no convincing evidence for the involvement of a specific one has been provided. We investigated the role of Apolipoprotein E receptor 2 (ApoER2) on the pathogenic effects of a patient-derived polyclonal aPL IgG preparation (IgG-APS), a murine anti-␤ 2 GPI monoclonal antibody (E7) and of a constructed dimeric ␤ 2 GPI I (dimer), which in vitro mimics ␤ 2 GPI-antibody immune complexes, using an animal model of thrombosis, and ApoER2-deficient (؊/؊) mice. In wild type mice, IgG-APS, E7 and the dimer increased thrombus formation, carotid artery TF activity as well as peritoneal macrophage TF activity/expression. Those pathogenic effects were significantly reduced in ApoER2 (؊/؊) mice. In addition, those effects induced by the IgG-APS, by E7 and by the dimer were inhibited by treatment of wild-type mice with soluble binding domain 1 of ApoER2 (sBD1). Altogether these data show that ApoER2 is involved in pathogenesis of antiphospholipids antibodies. (Blood. 2011;117(4):1408-1414) IntroductionThe association between persistently present antiphospholipid (aPL) antibodies and the clinical manifestations of thrombosis or pregnancy morbidity is known as the antiphospholipid syndrome (APS). 1 aPL antibodies are heterogeneous and recognize a wide variety of plasma proteins with phospholipid-binding properties, such as prothrombin 2 and ␤ 2 glycoprotein I (␤ 2 GPI). 3,4 aPL antibodies directed against ␤ 2 GPI, a plasma protein without known physiologic function, are considered the most pathologically relevant antibodies.There is strong experimental evidence that anti-␤ 2 GPI antibodies have thrombogenic properties. In studies on endothelial cell activation, 5-8 authors have shown the induction of a prothrombotic and proinflammatory phenotype upon exposure to anti-␤ 2 GPI antibodies, indicated by expression of tissue factor (TF) and increased surface expression of adhesion molecules, such as intercellular adhesion molecule-1, vascular-cell adhesion molecule-1, and E-selectin. Activation of monocytes by anti-␤ 2 GPI antibodies leads to TF expression as well. 9 Furthermore, anti-␤ 2 GPI antibodies, or recombinant dimers of ␤ 2 GPI that mimic ␤ 2 GPI-antibody immune complexes, increase platelet deposition to extracellular matrix components in in vitro flow models. 10 Injection of anti-␤ 2 GPI antibodies in murine 11 or hamster 12 thrombosis models leads to increased thrombus formation.Several receptors were postulated to mediate the prothrombotic cellular effects of anti-␤ 2 GPI antibodies. The interaction between annexin A2 and ␤ 2 GPI-antibody immune complexes has been reported to lead to endothelial cell activation. 13 It seems unlikely, however, that annexin A2 is able to convey activation signals across the cell membrane because this phospholipid-binding protei...

Summary. Objectives: In the antiphospholipid syndrome (APS), the immunodominant epitope for the majority of circulating pathogenic antiphospholipid antibodies (aPLs) is the N-terminal domain I (DI) of b 2 -glycoprotein I. We have previously shown that recombinant DI inhibits the binding of aPLs in fluid phase to immobilized native antigen, and that this inhibition is greater with the DI(D8S/D9G) mutant and absent with the DI(R39S) mutant. Hence, we hypothesized that DI and DI(D8S/D9G) would inhibit aPL-induced pathogenicity in vivo. Methods: C57BL/6 mice (n = 5, each group) were injected with purified IgG derived from APS patients (IgG-APS, 500 lg) or IgG from normal healthy serum (IgG-NHS) and either recombinant DI, DI(R39S), DI(D8S/D9G), or an irrelevant control peptide (at 10-40 lg). Outcome variables measured were femoral vein thrombus dynamics in treated and control groups following standardized vessel injury, expression of vascular cell adhesion molecule-1 (VCAM-1) on the aortic endothelial surface, and tissue factor (TF) activity in murine macrophages. Results: IgG-APS significantly increased thrombus size as compared with IgG-NHS. The IgG-APS thrombus enhancement effect was abolished in mice pretreated with recombinant DI (P £ 0.0001) and DI(D8S/D9G) (P £ 0.0001), but not in those treated with DI(R39S) or control peptide. This inhibitory effect by DI was dose-dependent, and at lower doses DI(D8S/D9G) was a more potent inhibitor of thrombosis than wild-type DI (P £ 0.01). DI also inhibited IgG-APS induction of VCAM-1 on the aortic endothelial surface and TF production by murine macrophages. Conclusion: Our findings in this proof-of-concept study support the development of recombinant DI or the novel variant DI(D8S/D9G) as a potential future therapeutic agent for APS.

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BackgroundDevelopment of thoracic aortic aneurysms is the most significant clinical phenotype in patients with Marfan syndrome. An inflammatory response has been described in advanced stages of the disease. Because the hallmark of vascular inflammation is local interleukin‐6 (IL‐6) secretion, we explored the role of this proinflammatory cytokine in the formation of aortic aneurysms and rupture in hypomorphic fibrillin‐deficient mice (mgR/mgR).Methods and ResultsMgR/mgR mice developed ascending aortic aneurysms with significant dilation of the ascending aorta by 12 weeks (2.7±0.1 and 1.3±0.1 for mgR/mgR versus wild‐type mice, respectively; P<0.001). IL‐6 signaling was increased in mgR/mgR aortas measured by increases in IL‐6 and SOCS3 mRNA transcripts (P<0.05) and in cytokine secretion of IL‐6, MCP‐1, and GM‐CSF (P<0.05). To investigate the role of IL‐6 signaling, we generated mgR homozygous mice with IL‐6 deficiency (DKO). The extracellular matrix of mgR/mgR mice showed significant disruption of elastin and the presence of dysregulated collagen deposition in the medial‐adventitial border by second harmonic generation multiphoton autofluorescence microscopy. DKO mice exhibited less elastin and collagen degeneration than mgR/mgR mice, which was associated with decreased activity of matrix metalloproteinase‐9 and had significantly reduced aortic dilation (1.0±0.1 versus 1.6±0.2 mm change from baseline, DKO versus mgR/mgR, P<0.05) that did not affect rupture and survival.ConclusionActivation of IL‐6‐STAT3 signaling contributes to aneurysmal dilation in mgR/mgR mice through increased MMP‐9 activity, aggravating extracellular matrix degradation.